1. Can you tnsping the DB service from the DAC server as the DAC OS process owner? If not, fix the issue or ask the DBA why you cannot.
2. Do you set up the proper .jar libraries (ojdbc6.jar file in the \lib directory on the client machine) when setting up DAC.
3. Did you follow the steps here: Section 4.7.4.1 How to Create ODBC Connections for Oracle